1. Field of the Invention
Exemplary aspects of the present disclosure generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof, and more particularly, to a power supply module that supplies a bias in which an alternating current voltage is superimposed on a direct current voltage to transfer a toner image onto a recording medium and an image forming apparatus including the power supply module.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member (which may, for example, be a photoconductive drum); an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member by a transfer electric field generated by a certain voltage such as a direct current (DC) voltage; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to affix the unfixed toner image on the recording medium semi-permanently, thus forming the image on the recording medium.
There is increasing market demand for an image forming apparatus capable of forming an image on various kinds of recording media sheets such as ones having a coarse surface, for example, Japanese paper and an embossed sheet. However, transferring a toner image onto a recording medium having a coarse surface using the transfer electric field generated by the DC voltage using the conventional configuration, a pattern of light and dark patches according to the surface condition of the recording medium appears in an output image. This is because the toner is transferred poorly to recessed portions on the surface of the recording medium, and as a result, the density of toner at the recessed portions is less than that of projecting portions of the recording medium.
In order to obtain an image without uneven toner concentration regardless of the surface condition of the recording medium, the transfer electric field can be generated using a superimposed bias in which an alternating current (AC) voltage is superimposed on a DC voltage. In this configuration, the AC-DC superimposed bias is applied to a secondary transfer member such as a secondary transfer roller. The AC-DC superimposed bias is composed of a DC voltage and an AC voltage in which a relatively high first peak-to-peak voltage and a relatively low second peak-to-peak voltage alternate. The transfer electric field generated by the AC-DC superimposed bias enables the toner image on the intermediate transfer belt serving as an image bearing member to move to the recording medium. Accordingly, unevenness of image concentration is reduced. The mechanism by which this feat is accomplished is as follows.
Initially, with application of a transfer bias composed of a superimposed bias at first only a small number of toner particles on the toner layer on the image bearing member separates from the toner layer and moves to the recording medium; most of the toner particles remain in the toner layer.
After the toner particles separated from the toner layer enter the recessed portions of the recording medium, the polarity of the transfer electric field reverses due to the AC voltage. As a result, the toner particles in the recessed portions return to the toner layer. When this happens, the toner particles returning to the toner layer strike the toner particles remaining in the toner layer, thereby weakening adhesion of the toner particles in the toner layer. Subsequently, when the polarity of the transfer electric field reverses towards the direction of the recording medium, more toner particles than the initial time separate from the toner layer and move to the recessed portions of the recording medium.
As this process is repeated, the amount of toner particles separating from the toner layer and entering the recessed portions of the recording medium can be increased, thereby transferring adequately the toner to the recessed portions of the recording medium.
However, although effective, in order to apply the AC-DC superimposed voltage, various components are required. For example, an AC power source for supplying the AC voltage, components that control the power source such as a signal line, and a harness that connects the AC power source and the transfer device are required.
Although an AC-DC superimposed bias is used to transfer a toner image onto a recording medium with a coarse surface as described above, the transfer electric field is generated using only the DC voltage (direct current bias) when forming an image on a normal sheet. In such a case, a switching mechanism such as a relay is required to switch between the biases to produce different transfer electric fields.
In known image forming apparatuses that use an AC-DC superimposed bias, arrangement of various constituent components to produce and control the AC-DC superimposed bias such as the AC voltage power source, harnesses, signal lines, and a relay is not discussed in detail. Yet in order to satisfy recent demand for overall size reduction of the image forming apparatus, arrangement of the constituent components is important. Furthermore, to reduce the time and the cost of assembly of the image forming apparatus, the constituent components need to be assembled easily. Hence, arrangement of the components is critical in this regard as well.
In addition, it is conceivable that users purchase an image forming apparatus without the components for application of the AC-DC superimposed bias but later wish to add these components optionally. In such a case, a technician needs to be called in to install the components required for application of the AC-DC superimposed bias. However, as is generally the case for the image forming apparatus, the power source and the like that are not expected to be touched or removed by the user are disposed at the back of the image forming apparatus. In order to attach the additional components for the AC-DC superimposed bias to the existing image forming apparatus, it may be necessary to move the image forming apparatus so that he or she can access the back of the image forming apparatus, which generally faces a wall of the office upon installation of these components.
As is obvious, if installation of the components in the image forming apparatus is time-consuming, downtime, that is, a period of time during which the device is not operated, also lengthens. Moreover, if installation of the components requires disassembly of the image forming apparatus to some extent, a relatively large working space is required, which is inconvenient for the user.
In view of the above, there is demand for an image forming apparatus that combines good imaging capability regardless of the surface condition of the recording medium with ease of installation of the components needed to generate the AC-DC superimposed bias.